The present invention relates to a method of adjusting a frequency response in a stripline filter device which may be used as a band-pass filter for example.
Such a stripline filter device is known, which is utilized as a band-pass filter for a microwave range. An example of such a conventional stripline filter device is illustrated in FIG. 1. As will be seen in FIG. 1, it comprises a lower dielectric substrate 1 and an upper dielectric substrate 2 which are stacked to each other. Each of the dielectric substrates 1 and 2 may be of dielectric ceramic material having a high dielectric constant and a lower dielectric loss such as BaO--TiO.sub.2, BaO--TiO.sub.2 -rare earth or the like. The lower dielectric substrate I is provided with an external ground conducting layer 3 on the peripheral portion and bottom surface thereof. Similarly, the upper dielectric substrate 2 is provided with an external ground conducting layer 4 on the peripheral portion and upper surface thereof. On the upper surface of the lower dielectric substrate 1 are disposed a plurality of stripline resonator conducting layers 5, 6 and 7 which form a filter element. Each resonator conducting layer has one end or an open circuit end (5a, 6a and 7a) spaced from the ground conducting layer 3 and the other end or a short circuit end (5b, 6b and 7b) connected to the ground conducting layer 3. The open circuit ends 5a, 6a and 7a of the respective resonator conducting layers 5, 6 and 7 are alternately disposed so as to form an interdigitated configuration. The upper dielectric substrate 2 is fixed on the lower dielectric substrate 1, and the ground conducting layers 3 and 4 of the respective dielectric substrates are connected to each other.
As well known in the art, the filter device of this type has a frequency response which depends on the configuration and dielectric constant of the substrates, and the dimension of the resonator conductors. Upon the manufacturing of the filter device the dielectric constant Of the substrates and the size of the resonator conducting layers are strictly determined. However, it can not be avoided that there may occur any dispersions in the dielectric constant of the substrates and in the dimension of the resonator conducting layers. It is, therefore, necessary to adjust the frequency response of the filter device after being completed.
The adjustment of the frequency response can not be performed by adjusting the length of the resonator conducting layers because they are embedded in the dielectric substrates. One solution to this problem has been proposed in U.S. Pat. No. 4,157,517. According to the adjusting method disclosed in this U.S. patent, the frequency of the filter is previously set at a lower level than a desired one, and the external conductor or ground conducting layer provided on the upper surface of the upper substrate is partially removed at regions adjacent the open circuit ends of the resonator conducting layers to reduce the capacitance between the external conducting layer and the respective resonator conducting layers and to increase the response frequency of the filter thereby making it possible to adjust the frequency.
This previously proposed adjusting method are extremely useful for the frequency response characteristic of the stripline filter device. However, as the number of the resonator conducting layers to be provided is increased, the frequency response characteristics of the respective resonator conducting layers intricately interact with each other, thus involving much difficulty for individually discerning and properly adjusting each frequency response characteristic.